A DESCRIPTION OF THE REQUEST :
JavaFx has only an external styledEditor RichTextFx https://github.com/FXMisc/RichTextFX

This one has massive memory problems. For one 5Mbyte file the Java memory usage increases up to 500 Mbytes. The software is written using a separate Text component for each keyword, which are layed-out on the screen using an hacked version of the TextFlow.

The javaFx HTMLEditor is much better, but missing specific features for a StyledTextEditor. This should support for : 
1. Style different keywords with a limited number of styles ( for example a style for keywords, comments, etc. )., similar with the editors used in Eclipse or Intellj ). 
2. For very large files should be possible not to style the document at all and still the memory usage to be fine.
3. Support for line number on the left


JUSTIFICATION :
RichTextFx has memory problems. See https://github.com/FXMisc/RichTextFX/issues/659 and few other issues on their website page.